LeetCode (Python): Single Number II

Given an array of integers, every element appears three times except for one. Find that single one.
Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory? 

Solution:

We keep tree variables: one with the bits that appear $3k+1$ times, two with those that appear $3k+2$, and three with $3k$, such that $k$ is an integer.

class Solution:
    # @param A, a list of integer
    # @return an integer
    def singleNumber(self, A):
        one = 0
        two = 0
        three = ~0
        for i in range(0, len(A)):
            nextOne = (one & ~A[i]) | (three & A[i])
            nextTwo = (two & ~A[i]) | ( one & A[i])
            nextThree = (three & ~A[i]) | (two & A[i])
            one = nextOne
            two = nextTwo
            three = nextThree
        return one